1. Field of the Invention
The present invention relates to an optical head that optically records information on an information-recording medium such as an optical disc or reproduces or deletes information therefrom, and to an information recording and reproduction apparatus that uses this optical head.
2. Related Art of the Invention
An optical head that uses a push pull signal to generate a tracking error signal is commonly used due to an advantage that it can be implemented in an optical system of a simple configuration. It, however, has a disadvantage that an offset occurs in a tracking error a signal as an objective moves in a tracking direction. A technique that compensates for this disadvantage is disclosed in Japanese Patent Application Laid-Open No. 8-306057 and Japanese Patent Application No. 7-280372.
The conventional techniques are described below with reference to the drawings. FIG. 18 shows a configuration of a first conventional technique that is an optical head in Japanese Patent Application Laid-Open No. 8-306057. In this figure, 74 is the optical axis of the optical head; 75 is a light source; 76 and 77 are half mirrors; 78 is an objective; 79 is an actuator that moves the objective in the X and Y directions; 80 is an optical disc that is an information-recording medium; 81 is a six-piece light-receiving element (In the figure, a sub-figure showing the locational relationship between the element 81 and the optical axis 74 and a sub-figure showing a detection state are shown in parallel); 81a to 81c are parting lines of the six-piece light-receiving element 81; 82a to 82f are divided light-receiving areas; 83 is a differential amplifier that is an operation circuit; 84 is a luminous flux on the six-piece light-receiving element 81; 88 is a focus control section; and 89 is a tracking control section.
Next, the operation of the optical head of the above configuration is described. Light from the light source 75 is reflected by the half mirror 76 and converged by the objective 78 on an information-recording surface of the optical disc 80 to form a light spot. Information tracks are formed on the optical disc 80, and their direction is perpendicular to the sheet of the drawing in FIG. 18. Reflected light from the optical disc 80 is transmitted through the objective 78 and half mirror 76 and separated into two luminous fluxes by the half mirror 77. Luminous fluxes reflected from the half mirror 77 are incident on the focus control section 88, and transmitted luminous fluxes enter the six-piece light-receiving element 81.
An addition provided by the connections in the figure and a differential operation performed by the differential amplifier 83 generate a tracking error signal, which is then guided to the tracking control section 89. The focus control section 88 detects a focus error signal and controls the actuator 79 so that light is converged on the information-recording surface of the optical disc. Based on the detected tracking error signal, the tracking control section 89 controls the actuator 79 in such a way that the light spot is guided to the center of the information tracks, and moves the objective 78 in the X-positive and -negative direction using the optical axis 74 of the optical head as a reference.
FIGS. 19(a) and 19(b) show the position of the luminous flux 84 on the six-piece light-receiving element 81; FIG. 19(a) shows that the objective 78 is located at a reference position, and FIG. 19(b) shows that the objective moves in the X-positive direction. Two shaded parts in the luminous flux 84 show area in which a zero-order diffracted luminous flux and a positive and a negative first order diffracted luminous fluxes, which are diffracted by the optical disc 80, interfere with each other.
In FIG. 19(a), the luminous flux 84 is symmetrical relative to the parting line 81a, so an output signal form the differential amplifier 83 is a tracking error signal without offset. On the other hand, in FIG. 19(b), the luminous flux 84 is moved to the right to lose its symmetry relative to the parting line 81a, and the area of the luminous flux contained in each of areas 82b, 82d and 82f increases while the area of the luminous flux contained in each of areas 82a, 82c, and 82e decreases. When a signal detected in each light-receiving area is represented by its area name and the tracking error signal is referred to as TE, the operation performed by the differential amplifier 83 is as follows: EQU TE=82a+82e+82d-k*(82b+82f+82c) [Equation 3]
wherein (k) is a correction coefficient. A push pull signal component appears in the areas 82c and 82d mainly containing the shaded interfering areas, whereas an offset component appears in the other areas mainly due to the movement of the luminous flux. Thus, by setting the correction coefficient (k) at an appropriate value, the operation in Equation 3 provides a tracking error signal in which an offset is corrected that is caused by the movement of the objective.
Next, a second conventional technique that is the optical head in Japanese Patent Application No. 7-280372 is described. The configuration of this optical head is similar to that of the first conventional technique except for the division of the light-receiving element. Thus, its configuration diagram is omitted and only the configuration of the multi-piece light-receiving element is described.
FIG. 20(a) is a top view of an eight-piece light-receiving element. Reference numerals 85a to 85c designate parting lines of the eight-piece light-receiving element 85; 86a to 86h are divided light-receiving areas; 87 is a luminous flux on the eight-piece light-receiving element 85; and the shaded part in the figure is an area that is not exposed to light.
The operation of the optical head according to the second conventional technique is almost the same as that of the optical head according to the first conventional technique. Thus, the description of the operation is omitted and only the features obtained by configuring the eight-piece light-receiving element as described above are explained. FIG. 20(b) schematically shows the distribution of the amount of light in the luminous flux 87 which is generated when the optical disc is tilted in the radial direction, wherein the magnitude of light intensity is represented by the density of diagonal lines. In this figure, the light intensity increases with increasing density of diagonal lines. This figure indicates that asymmetrical light intensity occurs at the center of the luminous flux due to the inclination of the optical disc in the radial direction, and this asymmetry of the intensity distribution results in an offset in the push pull signal. If a shaded portion is present in the center as shown in FIG. 20(a), the effect of the asymmetrical light intensity shown in FIG. 20(b) can be reduced. When a signal detected in each area is represented by the corresponding area name, the tracking error signal TE can be obtained by the following equation: EQU TE=86c+86e-(86d+86f)-k*{86a+86g-(86b+86h)} [Equation 4]
wherein (k) is a correction coefficient.
As described above, the first conventional technique can correct an offset in the tracking signal caused by the movement of the objective in the tracking direction, and the second conventional technique can reduce an offset in the tracking signal caused by the inclination of the optical disc in the radial direction.
The conventional techniques, however, divide a luminous flux on the light-receiving element to detect a possible tracking error, so separate optical systems are required to detect a focus and a tracking error signals. Consequently, despite the simple configuration of the detecting optical system of a tracking error, it is difficult to miniaturize or integrate the optical head.